1. Field of Invention
This invention relates to apparatus for degassing floating solids containing entrapped gas an open-topped liquid-solid settling chamber wherein floating solids tend to accumulate at the surface of liquid being separated from solids.
2. Description of the Prior Art
In the conventional activated sludge systems in use today, wastewater is subjected to the usual screening and pretreatment steps, e.g., primary sedimentation, then mixed with recycled activated sludge to form a mixed liquor (liquid-solid mixture) which is subjected to aeration with an oxygen-containing gas in an aeration zone. During aeration of the mixed liquid, the microorganisms present in the activated sludge cause the aerobic decomposition of solids and a high degree of BOD removal is achieved.
Phosphates, which are present in organic wastes and detergents, escape conventional wastewater treatment processes and are released with the effluent into natural water resources, e.g., lakes, rivers and streams. These phosphates result in over-fertilization or eutrophication of water causing unsightly algal blooms and serious pollution problems.
It is known that aeration of the mixed liquor in an activated sludge wastewater treatment process initially causes the microorganisms present to take up phosphate. U.S. Pat. No. 3,236,766 discloses a process which utilizes this phenomenom for removing phosphates from wastewater. According to the process disclosed in that patent, the pH of the raw wastewater is adjusted, if necessary, to maintain a range from about 6.2 to about 8.5, the wastewater is mixed with activated sludge to form a mixed liquor, the mixed liquor is aerated to maintain a dissolved oxygen content of at least 0.3 mg. per liter in the mixed liquor and a phosphate-enriched sludge is separated from the mixed liquor to provide a substantially phosphate-free effluent. The phosphate-enriched sludge liquid-solid is treated to reduce the phosphate content thereof prior to recycling for mixing with the influent wastewater. This is accomplished by maintaining the phosphate-enriched sludge in an anaerobic condition for several hours in a combination phosphate stripper and sludge thickener vessel. In this vessel, the phosphate-enriched sludge is settled and thickened and the anaerobic conditions cause the microorganisms which took up phosphate in the aeration zone to release phosphates to the liquid phase to form a phosphate-enriched supernatant. The phosphate-enriched supernatent is delivered to a phosphate precipitator where a phosphate precipitating reagent such as lime is added to precipitate the soluble phosphates.
U.S. Pat. No. 4,042,493 discloses a specific improvement in the process of the aforedescribed U.S. Pat. No. 3,236,766. In accordance with the specific improvement features of the process described in the former patent, released phosphate is counter-currently stripped from the sludge in the stripping zone. Such improvement requires a sludge residence time in the stripping zone of from two to ten hours. A low phosphate, low solid stripping medium is introduced into the stripping zone lower section for upflow through at least part of the settling solids to the stripping zone upper section. In this manner the phosphate released from the settling sludge solids is transferred to the upflowing liquid to provide phosphate-enriched liquid in the stripping zone upper section. The stripping medium has a suspended solids content not exceeding 200 mg./liter and the volumetric flow rate of the stripping medium introduced into the stripping zone is maintained between 0.7 and 2.0 times the volumetric flow rate of the phosphate-enriched liquid withdrawn therefrom.
In the phosphate stripping chambers employed in the above-described processes, it has been found that floating sludge solids tend to accumulate at the surface of liquid being separated from sludge solids, due to the onset of nitrification in the sludge solids being treated. In the anaerobic stripping zone, nitrosomonas and nitrobacter bacteria develop under favorable conditions, convert the ammonia contained in the sludge solids to chemical species containing the nitrate radical and under the anaerobic conditions present other bacteria in the sludge solids will satisfy their oxygen requirement by reducing the nitrates thereby releasing free nitrogen. Such biochemical reactions thus result in the release of nitrogen in the biological solids in the phosphate stripping zone thereby causing flotation of sludge containing entrapped gas, with resulting carryover of sludge solids in the phosphate-enriched supernatant liquid withdrawn from the stripping zone. Such carryover of floating sludge solids is severely detrimental to process efficiency for several reasons. First, the floating sludge is unsightly and can cause objectionable odors. Secondly, the sludge also contains acid chemical species and, therefore, in the presence of such floating sludge solids, the chemical reagent requirement for phosphate precipitation from the supernatant liquid is correspondingly increased. Further, the chemically precipitated phosphate sludge, i.e., the phosphate precipitant precipitated from supernatant liquid withdrawn from the stripping zone, is often used as crop fertilizer, and when contaminated with floating bacterial sludge solids, it is less acceptable for such use. Finally, sludge solids carryover in the supernatant liquid withdrawn from the phosphate stripping zone represents a loss of biological sludge solids, with possible adverse effect on the phosphate removal capability of the overall process.
The problem of floating sludge solids in a sludge settling chamber is not limited to the phosphate stripping chambers in the processes just described. Nitrification of sludge can also be encountered in wastewater treatment activated sludge plants which practice extended aeration or otherwise provide favorable conditions for the growth of nitrosomonas and nitrobacter bacteria. The biochemical reduction of the nitrates which are inevitably formed in the presence of such bacteria and subsequent release of nitrogen gas can also occur in the oxygen-deficient conditions which can develop in the settled solids layer of the clarifier or sludge thickener of such wastewater treatment plants. In these cases, the resulting floating sludge solids problem can be at least partially overcome by increasing the underflow rate from the clarifier or thickener, thereby reducing the sludge retention time in the sludge blanket. This is due to the fact that for nitrification, i.e., conversion of ammonia to nitrates, to occur, biological sludge solids must be retained in the sludge for a significant period of time to sustain adequate population of the slow-growing nitrifying bacteria. Nonetheless, such increase in the underflow rate from the clarifier or settling chamber reduces the sludge thickening capability of the clarifier or thickener, with the attendant disadvantages that are associated with the reduced level of solids in the settled sludge underflow.
Accordingly, it is an object of the present invention to provide apparatus for degassing floating solids containing entrapped gas in an open-topped liquid-solid settling chamber wherein floating solids tend to accumulate at the surface of liquid being separated from solids.
Other objects and advantages of the present invention will be apparent from the ensuing disclosure and appended claims.